As the Omicron-driven surge in Covid-19 infections keeps apace aggressively, Massachusetts is also grappling with the early months of what is shaping up to be an active time for the seasonal flu.

Influenza-like illness activity reached the highest of four severity ratings in the week from Dec. 19 to Dec. 25, the most recent period with a Department of Public Health report available. That’s the earliest point in the season that the Bay State has hit that level in several years.

The 93 health care facilities that serve as “sentinel sites” to monitor the flu reported that 3.38 percent of visits they saw in the week ending on Christmas Day were for influenza-like illness, well above the 2 percent regional baseline that estimates a typical impact and higher than the same week in any of the past three years.

“Many more people are infected with influenza than are tested for influenza,” the department noted in its report. DPH defines influenza-like illness (ILI) as a fever of at least 100 degrees Fahrenheit in addition to a cough and/or a sore throat — symptoms similar to those that Covid patients experience.

While activity has been robust so far, the intensity of influenza illness has not climbed at the same rate. DPH called the severity of influenza “moderate,” the second lowest of four options, in the week ending Dec. 25.

The federal Centers for Disease Control and Prevention (CDC) cautioned of “early signs that flu vaccination uptake is down this season compared to last.” The flu season “is just getting started,” the national agency wrote. “There’s still time to get vaccinated. An annual flu vaccine is the best way to protect against flu and its potentially serious complications. CDC recommends everyone six months and older get a flu vaccine.”

About 0.62 percent of hospitalizations in Massachusetts were associated with influenza the week ending Dec. 25, in line with the same point during the 2019-2020 season and below this time in the 2018-2019 season.

So far, the flu’s effect has varied by region.

DPH said central Massachusetts is experiencing the most potent impact, while the northeast, southeast and inner metro Boston regions also face high influenza-like illness activity. Boston itself, the outer metro Boston area and western Massachusetts are reporting moderate influenza-like illness activity, DPH said.

DPH’s most recent report, published last Friday, also marks the first time that state health officials have deemed influenza-like illness activity high since the week ending April 18, 2020, early on in the pandemic that has upended the world.

For the entire October 2020 to May 2021 season, when pandemic precautions such as capacity restrictions and social distancing were more common, DPH said its sentinel providers reported “minimal” flu activity.

In the 2019-2020 season, DPH declared flu activity high across the state for the first time the week ending Dec. 28, 2019.

Most positive influenza specimens recently reported by health care facilities in Massachusetts are influenza Type A, and H3N2 is currently the predominant strain, DPH said.

As they prepare for an uptick in flu illness, health care providers in Massachusetts and across the country are already struggling under the weight of the ongoing Omicron surge and persistent staffing shortages. A group of physicians and nurses cautioned last week the state’s emergency departments are at “critical capacity” and that “things will get worse.”

The flu season also continues to ramp up in other parts of the country. In its own report covering the week ending Dec. 25, the US Centers for Disease Control and Prevention said the eastern and central United States are “seeing the majority of viruses reported” while western states are reporting lower levels of influenza virus circulation.

Most influenza A (H3N2) infections have occurred among children and young adults between the ages of 5 and 24 years old, the CDC said, but the “proportion of infections occurring among adults age 25 years and older has been increasing.”

The CDC deemed Massachusetts and 15 other states and jurisdictions to have “high” influenza-like illness activity in the most recent week of surveillance. Three others — Georgia, North Dakota, and New Jersey — had “very high” activity in that span, the CDC said.